=404. Complementary Colors.=--If two prisms are placed in reversed position near each other (see Fig. 401), a beam of light dispersed by one is recombined into white light by the other. If now a card is held between the two prisms so as to cut off some of the colored light, say the red, the remaining light will be found to form a _greenish blue_. If the card is removed, the light becomes _white_ again. That is, red and _peacock blue_ light together form white. Any two colors that together form white light are called _complementary_. Other complementary colors are light yellow and blue, green and crimson, orange and greenish blue, violet and greenish yellow. We must not confuse the combining of colors (light) and the combining of _pigments_, the latter consisting of bodies that absorb light. Yellow pigment absorbs all but yellow and some green, while blue pigment absorbs all but blue and some green. Mixing these two pigments causes the absorption of all colors but _green_. Blue and yellow _paint_ mixed produce _green_, while blue and yellow _light_ give white. =405. The solar spectrum=, as the spectrum of sunlight is called, may be observed in the _rainbow_. The latter is produced through the dispersion of light by spherical raindrops. Its formation may be imitated by sending a small circular beam of light through a screen against a round glass flask filled with water. (See Fig. 404.) The light passes through the water and is dispersed when it enters and when it leaves, producing a color upon the screen at _R_-_V_. The course of the light within the drop is indicated in Fig. 405. The violet ray comes to the eye more nearly horizontal and is therefore below red, as we look at the rainbow. =406. Fraunhofer Lines.=--Some of the most important features of the solar spectrum are not seen in the rainbow or in the band of light usually observed upon a screen. By the use of a narrow slit and a convex lens to carefully focus the slit upon a white screen it is seen that the solar spectrum is crossed by many _dark_ lines. These are called Fraunhofer lines, to honor the German scientist who in 1814 first accurately determined _their_ position. Two experiments _with a spectroscope_ will help to make clear the meaning of the Fraunhofer lines. [Illustration: FIG. 404.--A rainbow formed by a beam of light striking a flask of water.] [Illustration: FIG. 405.--The course of a beam of light within a drop of water.] =407. The Spectroscope and Its Uses.=--The spectroscope (Fig. 406) is an instrument for observing spectra. It consists of a prism, a slit, and a convex lens _T_ for focusing an image of the slit accurately upon a screen (Fig. 407) where the spectrum is observed through the eyepiece _E_. [Illustration: FIG. 406.--The spectroscope.]